Spatial response of coastal marshes to increased atmospheric CO ₂

Abstract

The elevation and extent of coastal marshes are dictated by the interplay between the rate of relative sea-level rise (RRSLR), surface accretion by inorganic sediment deposition, and organic soil production by plants. These accretion processes respond to changes in local and global forcings, such as sediment delivery to the coast, nutrient concentrations, and atmospheric CO 2 , but their relative importance for marsh resilience to increasing RRSLR remains unclear. In particular, marshes up-take atmospheric CO 2 at high rates, thereby playing a major role in the global carbon cycle, but the morphologic expression of increasing atmospheric CO 2 concentration, an imminent aspect of climate change, has not yet been isolated and quantified. Using the available observational literature and a spatially explicit ecomorphodynamic model, we explore marsh responses to increased atmospheric CO 2 , relative to changes in inorganic sediment availability and elevated nitrogen levels. We find that marsh vegetation response to foreseen elevated atmospheric CO 2 is similar in magnitude to the response induced by a varying inorganic sediment concentration, and that it increases the threshold RRSLR initiating marsh submergence by up to 60% in the range of forcings explored. Furthermore, we find that marsh responses are inherently spatially dependent, and cannot be adequately captured through 0-dimensional representations of marsh dynamics. Our results imply that coastal marshes, and the major carbon sink they represent, are significantly more resilient to foreseen climatic changes than previously thought.sea-level rise | coastal marshes | coastal dynamics | atmospheric CO 2 | CO 2 fertilization C oastal marsh extent and morphology are directly controlled by rate of relative sea-level rise (RRSLR) and the soil accretion rate, the latter associated with inorganic sediment deposition and organic soil production by plants. Previous studies observed that CO 2 fertilization increases marsh plant biomass productivity through increased water use efficiency and photosynthesis (1), and hypothesized that, as a consequence, marsh resilience should increase via increased organic accretion (2, 3). However, this hypothesis has not yet been tested, and the observed increased plant productivity in response to the CO 2 fertilization effect has not been translated into its actual geomorphic effects. In fact, direct CO 2 effects on vegetation and marsh accretion (as opposed to its indirect effects, e.g., via the increase in temperature) have not yet been incorporated into marsh models, and their importance relative to other leading forcings of marsh dynamics (e.g., inorganic deposition, RRSLR, nutrient levels) remains unknown. Here we use existing data and a 1D ecomorphodynamic model to assess the direct impacts of elevated CO 2 on marsh morphology, relative to ongoing [e.g., RRSLR, and suspended sediment concentration (SSC)] and emerging [nutrient levels (4-6)] environmental change. Vegetation Responses to Changing Environmental ConditionsWe use publ...

Thresholds of sea‐level rise rate and sea‐level rise acceleration rate in a vulnerable coastal wetland

“…When we account for the medium fertilization effect of higher CO 2 concentration, that is, 39% increase in above-ground productivity and 33% increase in below-ground productivity by 2100, we find the threshold of SLR rate increases from 8.4 mm/year to 10.3 mm/year for 2100. The magnitude of increase in the SLR rate threshold accounting for medium fertilization effect of CO 2 is larger than the results in Ratliff, Braswell, and Marani, (2015), stressing the importance of vegetation productivity on the deposition process and increasing the resilience of coastal wetlands to SLR at the Grand Bay NERR. Even with the higher threshold of SLR rate by the CO 2 fertilization effect, the accumulated SLR by 2100 still falls within the likely and very likely ranges of SLR under the RCP8.5 scenario, but is much larger than the likely and very likely range of SLR under the low emission scenario (Horton, Rahmstorf, Engelhart, & Kemp, 2014;IPCC, 2013).…”

Feedback of coastal marshes to climate change: Long‐term phenological shifts

“…The marsh areas are estimated in 1988. Although the enhancement of plant growth from CO 2 , as discussed earlier, is mainly for C3 plants, the fertilization of CO 2 , based on a modeling study, can increase the marsh elevation for a mixed C3 and C4 plant community (by increasing plant production) in similar magnitude to the effect of increasing inorganic sediment input (Ratliff et al, 2015). Hence, the negative correlation between the marsh area and sea-level can be expected.…”

“…The three key phenological parameters estimated are peak NDVI (Panel a), peak NDVI day (Panel b), and growing season length (bracketing days that had NDVI greater than 90% of peak NDVI; Panel c). The elevated air temperature and atmospheric CO 2 may promote the marsh plant growth (for both aboveground and belowground) and thus their organic accretion (Langley, McKee, Cahoon, Cherry, & Megonigal, 2009;Ratliff, Braswell, & Marani, 2015). Sea-level rise may decrease the marsh elevation by reducing their organic accretion (Nyman, Delaune, Roberts, & Patrick, 1993), promoting decomposition of the marsh substrates (Craft, 2007;Weston, Vile, Neubauer, & Velinsky, 2011;Stagg, Schoolmaster, Krauss, Cormier, & Conner, 2017), and increasing erosion (Smith, Cialone, Wamsley, & McAlpin, 2010).…”

“…Hence, the negative correlation between the marsh area and sea-level can be expected. It should also be noted that the influence of CO 2 fertilization on marsh elevation is likely to depend on many other factors such as the inorganic sedimentation rate (Langley & Megonigal, 2010;Langley et al, 2009;Ratliff et al, 2015) and may be offset by the enhanced decomposition resulted from a rising air temperature (Charles & Dukes, 2009;Kirwan & Blum, 2011). The elevated air temperature and atmospheric CO 2 may promote the marsh plant growth (for both aboveground and belowground) and thus their organic accretion (Langley, McKee, Cahoon, Cherry, & Megonigal, 2009;Ratliff, Braswell, & Marani, 2015).…”

Hyperspectral and Multispectral Retrieval of Suspended Sediment in Shallow Coastal Waters Using Semi-Analytical and Empirical Methods

“…The suspended sediment concentration (SSC) largely determines water turbidity, a chief control of coastal ecosystem dynamics [1]. Furthermore, sediment supplied by rivers and tidal currents is essential to the survival of intertidal (i.e., saltmarshes) and sub-tidal structures (i.e., tidal flats) as sea level rise accelerates [2][3][4][5][6][7][8]: sediment "starvation" is one of the main sources of ongoing coastal degradation [9][10][11][12][13]. Consequently, methods for monitoring SSC in a spatially-distributed manner are vital for understanding and managing coastal systems.…”